1. Field of the Invention
The present invention relates to an interpolation equipment for pictorial signals, etc., which have been obtained by offset sampling.
2. Related Background Art
Conventionally, a method called offset sampling has been known as a method to compress the band or reduce information amount in the recording and transmission of various kinds of information signals such as image signals.
This offset sampling, in the case of two dimensions, is such that as shown in FIG. 1, the sampling interval (Tx, Ty) between the horizontal direction (x direction) and the vertical direction (y direction) is set twice the pixel interval at the original signals (Hx, Hy) and at the same time the adjacent points of sampling in the vertical direction are offset by one-half the sampling interval (Tx/2). The transmission band by the offset sample can be made wider, as shown in FIG. 2, in terms of spatial frequency component in the horizontal or vertical direction against the spatial frequency of a diagonal direction. Also, in case the image signal of the above offset sampling is displayed on the monitor or printed out, it is necessary, as shown in FIG. 3, to give interpolation processing for interpolating pixels at each sampling point (interpolation pixel) with adjacent pixels. Such interpolation processing functions as a spatial filter which allows the passing of frequency components in the shaded part of FIG. 2 and also inhibits the passing of frequency components in the area containing a turning back point A. This interpolation processing is positioned as in after-filter in the sampling theory.
Meanwhile, such an offset sampling as mentioned above is a very effective method if the prefilter before the sampling is correctly positioned, namely if a sufficient band limitation is made. However, if the pre-filter positioning is not appropriate owing to, for example, hardware limitation or if the pre-filter is not given thoroughly to widen the transmission band, there arises a problem of image deterioration owing to the generation of turning back distortion.
In other words, if the offset sampling is made when the pre-filter is insufficient, the high-area component of the original signal in the horizontal direction, as shown in FIG. 4A, turns back as a high-area component of the vertical direction and at the same time, as shown in FIG. 4B, the high-area component of the vertical direction turns back as the high-area component of the horizontal direction.
For this reason, the offset sampling is made as shown in FIG. 5B without sufficient band limitation of pictures containing the linear portion as shown in FIG. 5A. Even if the above-mentioned interpolation processing is made to the sampling signal containing this turning-back component, the high-area component at the original signal and the high-area component of the turning back cannot be separated, so that the turning-back distortion cannot be removed entirely. Thus, there arises uneveness of shade or blot on the linear part as shown in FIG. 5C, resulting in pictorial deterioration of an output picture.
Accordingly, an adaption-type interpolation method has generally been known as a method to mitigate the generation of the above turning-back distortion.
This adaption-type interpolation method detects, in the interpolation processing of the sampled image signals, whether the interpolation should be started with the pixels of the horizontal direction, whether it should be the pixels of the vertical direction or whether all of the surrounding pixels should be interpolated. Based on the result of this detection, spatial filters having such characteristics as shown in FIGS. 6A-6C are used selectively to remove unnecessary turning-back components. In this event, if the interpolation pixels are those constituting the linear line of the horizontal direction, filters having such characteristics shown in FIG. 6A as allowing more frequency components of the horizontal direction to pass through, are used to filter (remove) frequency components of the vertical direction containing turning-back components.
Incidentally, in the case of the above adaption-type interpolation method, the accuracy of judgment to select interpolation means determines the performance of the interpolation equipment.
In other words, if the occurrence of false judgment has high probability, there was a problem in that the interpolation processing based on false detection not only reduced original signal components but also resulted in the increase of turning-back distortion.